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BGP Extension for Distributing CP Threshold Constraints of SR Policy
draft-liu-idr-bgp-sr-policy-cp-threshold-02

Document Type Active Internet-Draft (individual)
Authors Yisong Liu , Changwang Lin , Yuanxiang Qiu
Last updated 2024-11-08
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draft-liu-idr-bgp-sr-policy-cp-threshold-02
IDR Working Group                                                Y. Liu
Internet Draft                                             China Mobile
Intended status: Standards Track                                 C. Lin
Expires: May 24, 2025                              New H3C Technologies
                                                                 Y. Qiu
                                                   New H3C Technologies
                                                       November 8, 2024

    BGP Extension for Distributing CP Threshold Constraints of SR Policy
                draft-liu-idr-bgp-sr-policy-cp-threshold-02

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   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.

Abstract

   This document defines the extension of BGP to distribute threshold
   and metric constraint parameters of candidate paths for SR Policy to
   achieve flexible path selection.

Table of Contents

   1. Introduction ................................................ 2
   2. Terminology ................................................. 3
   3. Carrying Constraint Parameters of CP in BGP ................. 3
   4. SR Bandwidth Constraint Sub-TLV ............................. 4
   5. SR Metric Constraint Sub-TLV ................................ 5
   6. Operations .................................................. 6
   7. IANA Considerations ......................................... 6
   8. Security Considerations ..................................... 7
   9. References .................................................. 7
      9.1. Normative References ................................... 7
      9.2. Informative References ................................. 8
   10. Acknowledgments ............................................ 8
   Authors' Addresses ............................................. 9

  1. Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at the ingress
   node. The ingress node steers packets into a specific path according
   to the Segment Routing Policy (SR Policy) as defined in [RFC9256].
   An SR Policy may have multiple candidate paths that are provisioned
   or signaled [I-D.ietf-idr-sr-policy-safi] [RFC8664] from one of more
   sources.

   [I-D.liu-spring-sr-policy-flexible-path-selection] proposes a
   flexible SR policy candidate path selection method. Based on the
   real-time resource usage and forwarding quality of candidate paths,
   the head node can perform dynamic path switching among multiple
   candidate paths in the SR policy.

   Multiple threshold parameters for SR Policy candidate path selection
   are listed in Section 4.1 of [I-D.liu-spring-sr-policy-flexible-
   path-selection]. This document defines extensions to BGP to
   distribute threshold and metric constraint parameters of candidate
   path (CP) for an SR Policy.

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  2. Terminology

   The definitions of the basic terms are identical to those found in
   Segment Routing Policy Architecture [RFC9256].

  3. Carrying Constraint Parameters of CP in BGP

   As defined in [I-D.ietf-idr-sr-policy-safi], a new SAFI is defined
   (the SR Policy SAFI with codepoint 73) as well as a new NLRI. The
   NLRI contains the SR Policy candidate path and, according to [I-
   D.ietf-idr-sr-policy-safi], the content of the SR Policy Candidate
   Path is encoded in the Tunnel Encapsulation Attribute defined in
   [RFC9012] using a new Tunnel-Type called SR Policy Type with
   codepoint 15.

   This document defines the following three Sub-TLVs to carry
   threshold and metric constraint parameters for candidate paths.

   * SR Bandwidth Constraint Sub-TLV

   * SR Metric Constraint Sub-TLV

   * SR Segment List Bandwidth Constraint Sub-TLV

   The new SR Policy encoding structure with Sub-TLVs of CP constraint
   parameters is expressed as below:

         SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>

            Attributes:

               Tunnel Encaps Attribute (23)

                  Tunnel Type: SR Policy (15)

                      Binding SID

                      SRv6 Binding SID

                      Preference

                      Priority

                      Policy Name

                      Policy Candidate Path Name

                      Explicit NULL Label Policy (ENLP)

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                      SR Bandwidth Constraint Sub-TLV

                      SR Metric Constraint Sub-TLV

                      Segment List

                          Weight

                          Segment

                          Segment

                          SR Segment List Bandwidth Constraint Sub-TLV

                          ...

                      ...

  4. SR Bandwidth Constraint Sub-TLV

   The SR Bandwidth Constraint sub-TLV is used to carry the bandwidth
   threshold constraint parameter of a candidate path.

   The SR Bandwidth Constraint sub-TLV is optional and it MUST NOT
   appear more than once in the SR Policy encoding.

   The format of the SR Bandwidth Constraint Sub-TLV is defined as
   follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |        RESERVED               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Bandwidth                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

   - Type: to be assigned by IANA

   - Length: Specifies the length of the value field. The value MUST be
      4.

   - Bandwidth: 4 octets which specify the bandwidth threshold in unit
      of bytes per second in IEEE floating point format.

   - RESERVED: 2 octet of reserved bits.  SHOULD be set to zero on
      transmission and MUST be ignored on receipt.

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  5. SR Metric Constraint Sub-TLV

   The SR Metric Constraint sub-TLV is used to carry the metric
   Constraint of a candidate path.

   The SR Metric Constraint sub-TLV is optional. Multiple instances of
   this sub-TLV may be used to carry different metric type uses.

   The format of the SR Metric Constraint Sub-TLV is defined as
   follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      | Metric Type   |     Flags     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Metric Margin                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Metric Bound                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

   - Type: to be assigned by IANA.

   - Length: Specifies the length of the value field. The value MUST be
      4.

   - Flags: 1-octet field that indicates the semantics of Metric Value.
      The following bit positions are defined and the other bits MUST be
      cleared by the originator and MUST be ignored by a receiver.

                0 1 2 3 4 5 6 7
               +-+-+-+-+-+-+-+-+
               |M|A|B|F|       |
               +-+-+-+-+-+-+-+-+
         where:

           *  M-Flag: Indicates that the metric margin allowed is
              specified when set.
           *  Flag: Indicates that the metric margin is specified as an
              absolute value when set and is expressed as a percentage
              of the metric when clear.
           *  Flag: Indicates that the metric bound allowed for the path
              is specified when set.
           *  F-Flag: Indicates that the Metric Margin and Metric Bound

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              are floating-point numbers when set to 1. When set to 0,
              it indicates they are integer numbers.
   - Metric Type: 1-octet field which identifies the type of the metric
      being used. The metric type code points are as follows:

           *  0: Bandwidth threshold in bytes per second.
           *  1: Weight threshold.
           *  2: Real-time bandwidth threshold in bytes per second.
           *  3: Round trip delay threshold in microseconds.
           *  4: Round trip loss threshold. The unit is 0.000003%.
   - Metric Margin: 4-octet value which indicates the minimum threshold
      when the M-flag is set. The metric margin is specified as either
      an absolute value or as a percentage of the path metric based on
      the A-Flag.

   - Metric Bound: 4-octet value which indicates the maximum threshold
      that is allowed when the B-flag is set. If the path metric crosses
      the specified bound value then the path is considered invalid.

  6. Operations

   The document does not bring new operation beyond the description of
   operations defined in Section 4 of [I-D.ietf-idr-sr-policy-safi].
   The existing operations defined in [I-D.ietf-idr-sr-policy-safi] can
   apply to this document directly.

   Typically, but not limit to, the SR policies carrying the constraint
   parameters of CP are configured by a controller.

   After configuration, the SR policies carrying the constraint
   parameters of CP will be advertised by BGP update messages.  The
   operation of advertisement is the same as defined in [I-D.ietf-idr-
   sr-policy-safi], as well as the reception.

  7. IANA Considerations

   This document defines a new sub-TLV in the registry "BGP Tunnel
   Encapsulation Attribute sub-TLVs" to be assigned by IANA:

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   +=======+==================================+=================+
   | Value | Description                      |  Reference      |
   +=======+==================================+=================+
   |  TBA1 | SR Bandwidth Constraint sub-TLV  |  This document  |
   +-------+----------------------------------+-----------------+
   |  TBA2 | SR Metric Constraint sub-TLV     |  This document  |
   +-------+----------------------------------+-----------------+

  8. Security Considerations

   [I-D.ietf-idr-sr-policy-safi] has discussed the security
   considerations for distributing SR Policy through BGP. This document
   does not introduce any new security issues.

9. References

  9.1. Normative References

   [I-D.ietf-idr-sr-policy-safi] Previdi, S., Filsfils, C., Talaulikar,
             K., Mattes, P., and Jain, D., "Advertising Segment Routing
             Policies in BGP", draft-ietf-idr-sr-policy-safi-02 (work
             in progress), March 2024.

   [I-D.liu-spring-sr-policy-flexible-path-selection] Liu, Y., Lin, C.,
             Peng, S., Mishra, G., and Qiu, Y., "Flexible Candidate
             Path Selection of SR Policy", draft-liu-spring-sr-policy-
             flexible-path-selection-05 (work in progress), February
             2024.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, DOI
             10.17487/RFC2119, March 1997, <https://www.rfc-
             editor.org/info/rfc2119>.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
             Decraene, B., Litkowski, S., and R. Shakir, "Segment
             Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
             July 2018, <https://www.rfc-editor.org/info/rfc8402>.

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   [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
             Hardwick, J., "Path Computation Element Communication
             Protocol (PCEP) Extensions for Segment Routing", RFC8664,
             DOI 10.17487/RFC8664, December 2019, <https://www.rfc-
             editor.org/info/rfc8664>.

   [RFC9256] Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
             P. Mattes, "Segment Routing Policy Architecture", RFC
             9256, DOI 10.17487/RFC9256, July 2022, <https://www.rfc-
             editor.org/info/rfc9256>.

  9.2. Informative References

   TBD

  10. Acknowledgments

   The authors would like to thank the following for their valuable
   contributions of this document:

   TBD

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Authors' Addresses

   Yisong Liu
   China Mobile
   Beijing
   China

   Email: liuyisong@chinamobile.com

   Changwang Lin
   New H3C Technologies
   Beijing
   China

   Email: linchangwang.04414@h3c.com

   Yuanxiang Qiu
   New H3C Technologies
   Beijing
   China

   Email: qiuyuanxiang@h3c.com

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